Abstract
In a previous study, we have shown that halide salts of the protonated Schiff base (pSB) of all-trans-retinal with aniline crystallize in the 6-s-trans conformation. These compounds are therefore better solid state models for the chromophores of the all-trans states of bacteriorhodopsin (bR) than the conventional butyl imine compounds. We have now extended these studies to the halide salts of the pSB of 13-cis-retinal with aniline. The 5-13C chemical shifts again indicate 6-s-trans conformations. These compounds are thus good models for the chromophores of the 13-cis photocycle intermediates of bR. As for the all-trans compounds, the frequencies of maximum visible absorbance and the 15N chemical shifts of the 13-cis compounds are linearly related to the strength of the pSB−counterion (CI) interaction as measured by (1/d 2), where d is the center-to-center distance between the pSB charge and the CI charge. However, the relationship is steeper for the 13-cis compounds. With these calibrations, we estimate that d = 4.0, 3.9, 3.7, 3.6, and 3.8 Å (±0.3 Å) for the J625, K590, L550, N520, and bR555 states of bR, respectively. These distances compare with similarly determined values of about 4.16 ± 0.03 and 4.66 ± 0.04 Å for the all-trans bR568 and O640 states, respectively. The results suggest that the pSB−CI interaction is stronger in all the 13-cis photocycle intermediates, including the red-shifted J and K intermediates, than in bR568. The apparent tightening of the pSB−CI interaction in the J → N sequence is consistent with models of the early photocycle in which electrostatic attractions between the pSB and counterions constitute an important constraint.